Tdoa based positioning with calculation of correction factors for compensating the clock offsets of unsynchronized network stations

ABSTRACT

The present invention presents a method, arrangement and computer program product for clocking exploiting the relative behavior of clocks of individual receiving stations as well as a corresponding modeling to derive a time difference of arrival of a signal from a user device which can be used to correct the time difference of arrival based on the modeled clock behavior and leads to a correct clocking of received user signals without the need of synchronization of the clocks in the various receiving stations. This principle is applicable to a plurality of pairs of receiving stations and beacon signals transmitted amongst them and allows for a correct location estimation of a user device.

The present invention relates to wireless mobile networks or accesspoints of a wireless local network having mobile user locationfunctions, associated methods and computer program products.

TECHNICAL BACKGROUND

In recent times it has become increasingly important to determine thelocation of a user using a mobile device in order to provide appropriateservices to users, such as restaurant or shop recommendation or tolocate the user in order to provide emergency medical services. On theother hand it has also become important to locate a user to provideappropriate law enforcement regarding criminal subjects.

For determining the location of a mobile device commonly satellitesupported methods have been established, such as using the GlobalPositioning System which is widely used to determine the position ofcars and used for route planning and navigation. Such methods howeverhave the disadvantage, that they require special receivers andtransmitters which are only suitable for navigational purposes andrequire an additional technical effort as well in the user device and interms of infrastructure to supply the corresponding satellites emittingproper navigation signals.

Due to the high competition in the mobile infrastructure and devicesmarket there is a strong tendency among the competitors to keep mobiledevices and their infrastructure technically simple, in order to remaincost-competitive on the market or to gain a competitive advantage.Therefore, there is a strong need to provide location services formobile devices which are technically simple while at the same time beingreliable and efficient as well as sufficiently accurate to determine theposition of a user in order to be able to provide to him or her suitableservices.

Accordingly methods have been established to determine the position of auser, by receiving a signal emitted by the user's mobile device at fixedreceiver stations, and based on the different signal propagation fromthe user device to the different fixed receivers to calculate a positionof the user device. In such an environment it is crucial that all thedevices involved in the location determination are basing theircorresponding analysis on a common clock. The positional accuracy ofsuch a system is directly related to the accuracy of the clocks and thecorresponding synchronicity of the clocks which are used to determinethe propagation delays involved in the location determination.

One such method is based on a time difference of arrival TDOA and usesthe time difference it takes for a signal to travel to two destinationsas an indirect method of calculating a distance. With a minimum of threebase stations, for instance, receiving the signal from a handset, thedifference in time it takes for the signal to reach each tower of a basestation can be used to triangulate the position of the mobile unit. TDOAsystems do not need any specialized antennas and as such theinfrastructure is kept simple. When a mobile that has to be locatedtransmits, the arrival time of the target mobile signal is recorded by aTDOA location measuring unit at each base station or access point whichis able to receive the signal. Since the mobile's signal travels at aconstant speed (the speed of light), comparison of the arrival time ofthe signal for any two sides allows a straightforward calculation todetermine the mobile's relative position to each side. When plotted,this relationship describes an imaginary hyperbola in space. The targetmobile is located somewhere on this curve although additionalinformation is required to determine precisely where. When the samecalculation is made involving measurements from a third base station oraccess point side, calculating a difference of arrival time from sitesA, B, C, e.g. between either sites A and C or between sites B and C, anindependent positional hyperbola can be described. The point at whichthe two hyperbolas AB and BC intersect is the location of the targetmobile. Commonly, TDOA requires accurate time synchronization at thebase station or access points, but not necessarily at the target mobiledevice. It is immediately apparent, that inaccuracies in the clockmeasurements may lead to large location errors. A high clock accuracyand the corresponding master clock and an associated synchronizationprocedure require however technically complicated solutions which on theother hand require adaptations at the infrastructure devices such asbase stations and for instance access points in order to provide themwith a suitable clock reference.

SUMMARY OF THE INVENTION

Therefore, there exists a need to provide a location estimation of amobile device without the requirement of clock synchronization orexpensive master clocks.

This problem is solved by a method for clocking according to claim 1, byan arrangement for clocking according to claim 13 or 15, and by acomputer program product for clocking according to claim 17.

Further advantageous developments of the invention are given in thedependent claims.

Advantageously the method according to the present invention exploitsthe fact that base stations of a wireless mobile network or accesspoints of a wireless local network are located at known positions andthus are situated in a fixed positional relationship. This allows tocalculate signal propagation delays of beacon signals transmittedamongst those stations and received individually. These are calculatedbased on the known signal propagation speed and the respective distancebetween the stations.

Additionally, determining the time difference of arrival allowsdetermination of the relation of local clocks of two individualreceiving stations to each other and requires no absolute values. Thus,by measuring the reception times of uniquely identified signals atindividual receiving stations, a relative clock behavior of pairs ofindividual receiving stations can be modeled and exploited together withthe absolute known time difference of arrival to calculate a correctionvalue for a signal received from a mobile device of a user by therespective base stations or wireless access points to correct thecorresponding time difference of arrival for the user signal.Consequently, the method according to the present invention solves theproblem of the present invention with using no additional hardware onlyby relating suitable calculations of suitable measurements at the accesspoints respectively base stations.

Expediently according to a further development of an example of themethod according to the present invention a transmission and a timestamping of a plurality of signals is analyzed which advantageouslyprovides for a higher accuracy in the model curve.

Beneficially according to a further development of the method of thepresent invention the signals are transmitted wirelessly over the air,which advantageously allows for a very simple infrastructure as nocables and wires need to be deployed.

Beneficially according to a further development of the method of thepresent invention the signal is embodied as a beacon signal, becausethis allows using normal wireless access points or base stations whichalready transmit beacon signals to be used in the context of the presentinvention.

Advantageously, according to a further development of the methodaccording to the present invention a signal is a frame, as frames bynature provide the advantage to possess a unique identification propertyin their frame number and thus further ease the implementation of themethod of the present invention in presently used transmission systems.

Further beneficially according to a development of the present inventionthe frame can be embodied as a frame according to a local area networkprotocol such as the IEEE 802.11 standard, as in this manner standardscan be easily adapted by the present invention and commonly usedtransmission standards are suitable for incorporation in the methodaccording to the present invention.

Advantageously for the modeling of the time difference of arrival of themethod according to the present invention according to an embodiment, apolynomial with least squares may be used because such a polynomial issimple in its mathematical structure while at the same time satisfyingthe descriptive needs of the dependency of two clocks according to themethod of the present invention without sacrificing any accuracy in themodeling process.

Expediently according to a further development of an embodiment of themethod according to the present invention at least one signal is beingtransmitted before the arbitrary point in time and one is beingtransmitted after the arbitrary point in time in order to improve theaccuracy of the clocking at the arbitrary point in time according to themethod of the present invention.

Beneficially according to a further development of the method accordingto the present invention it is ensured that the same number of signalshas been transmitted before the arbitrary point in time and after thearbitrary point in time to provide the utmost accuracy in thedetermination of the clocking according to a further embodiment of themethod according to the present invention.

Expediently according to a further development of the method of thepresent invention the timing difference of arrival at a different pairof receiving stations is used to be able to accurately determine thelocation of a mobile station by a method of time difference of arrivalbased location determination. This allows the determination of twohyperbolas and their corresponding intersection point as the location ofthe mobile device.

The present invention provides an arrangement for clocking comprising:

at least a first, a second and a third device;

the first device having transmitting means for transmitting at least afirst and a second signal with a unique identification;

the second and third devices having receiving means and a clock forreceiving the at least first and second signals and processing means forassociating a respective measured second and third local clock at thereception time of the respective signal to it, leading to at least twovalue pairs of local clocks of the second and third device;

modeling means for modeling based on the value pairs of local clocks atime function of a dependency of the second and third local clock overtime as a first model curve;

the second and third device further being adapted to receiving at anarbitrary point in time a user signal from a user device at the secondand third device and associating a reception time measured by therespective second and third local clock of the second and third deviceto the user signal upon its reception, leading to a user value pair oflocal clocks of the second and third device,

processing means for calculating a reference time difference of arrivalRTDOA for a signal from the first device received at the second andthird device from the fixed positional relationship, based on therespective distance between the first and the second device and thefirst and the third device and the known signal propagation speed; andfor calculating a user time difference of arrival UTDOA from the uservalue pair;

means for determining at the arbitrary point in time the value form thefirst model curve and based on this value calculating a determined timedifference of arrival DTDOA;

wherein the processing means is adapted to relate the RTDOA and DTDOA todetermine a current correction factor; and

for clocking to use the current correction factor to correct the UTDOA.

Beneficially the arrangement according to the present inventioncomprises a minimum number of receiving devices to achieve the clockingaccording to the present invention which provides a simpleinfrastructure in a competitive manner capable to solve the problem ofthe present invention.

Advantageously, a further development of the arrangement according tothe present invention allows employing a server for the computationalintensive tasks that need to be performed, which on the other handallows a further simplification of the receiving stations, and onlyrequires corresponding communications of the respective identificationof the received signals together with their associated time informationto the computation server for calculating a location of the mobiledevice.

Advantageously the computer program product of the invention provides asimple means to implement the method of the present invention atrespective base stations and access points by providing a means ofstorage and transport.

BRIEF DESCRIPTION OF THE DRAWINGS

Subsequently the invention will further be explained by means ofexamples and embodiments shown in drawings, wherein

FIG. 1 shows a simple transmitter and receiver arrangement according toan embodiment of the present invention;

FIG. 2 gives an example of a relative clock behaviour of two receivingstations;

FIG. 3 shows an example of signal transmission according to anembodiment of the present invention;

FIG. 4 shows a flow-chart explaining the estimation of a user locationbased on a time difference of arrival according to an embodiment of thepresent invention;

FIG. 5 gives an example of a computer program product according to thepresent invention; and

FIG. 6 gives an example of a device of an arrangement according to thepresent invention.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

The present invention will be described with respect to particularembodiments and with reference to certain drawings but the invention isnot limited thereto but only by the claims. The drawings described areonly schematic and are non-limiting. In the drawings, the size of someof the elements may be exaggerated and not drawn on scale forillustrative purposes. Where the term “comprising” is used in thepresent description and claims, it does not exclude other elements orsteps. Where an indefinite or definite article is used when referring toa singular noun e.g. “a” or “an”, “the”, this includes a plural of thatnoun unless something else is specifically stated.

The term “comprising”, used in the claims, should not be interpreted asbeing restricted to the means listed thereafter; it does not excludeother elements or steps. Thus, the scope of the expression “a devicecomprising means A and B” should not be limited to devices consistingonly of components A and B. It means that with respect to the presentinvention, the only relevant components of the device are A and B.

Furthermore, the terms first, second, third and the like in thedescription and in the claims, are used for distinguishing betweensimilar elements and not necessarily for describing a sequential orchronological order. It is to be understood that the terms so used areinterchangeable under appropriate circumstances and that the embodimentsof the invention described herein are capable of operation in othersequences than described or illustrated herein.

Moreover, the terms top, bottom, over, under and the like in thedescription and the claims are used for descriptive purposes and notnecessarily for describing relative positions. It is to be understoodthat the terms so used are interchangeable under appropriatecircumstances and that the embodiments of the invention described hereinare capable of operation in other orientations than described orillustrated herein.

As an example given in FIG. 1, a basic configuration to embody themethod of the present invention comprises or consists of three accesspoints AP1, AP2 and AP4. Without limiting the invention these accesspoints can be any stations that are capable of transmitting respectivelyreceiving signals, e.g. wireless signals. The potential to be atransceiver, i.e. as well a transmitter as a receiver has certainadvantages in terms of not having to have separate transmitting andreceiving stations and thus providing an optimum use of resources. Inthis case the access points may be access points of a wireless LAN, e.g.according to the IEEE Standard 802.11. But without any limitation theaccess points may be stations that are capable of transmittingrespectively receiving a signal, respectively a number of signals,especially wireless signals which can be provided with a uniqueidentifier or at least an element that enables a signal to beidentifiable as a unique signal. It is for instance conceivable thateach signal has a different format and thus the unique identifier of thesignal to be a unique signal is the format.

In this case the station AP4 transmits a unique signal, respectively asequence of unique signals that is/are received by stations AP2 and AP1.Upon reception of the unique signal or signals, by means of their localclock, the respective stations AP1 and AP2 associate the unique signalwith a time measured by their local clock that has been measured at thetime of reception of the unique signal. Without any limitation such anassociation may be performed for one, two or a plurality of uniquesignals in order to protocol a dependency of the local clocks of thestations AP2 and AP1 in association to a joint reception of the sameindividual unique signal from the station AP4.

Often at wireless access points, such as the stations AP1, AP2 and AP4,these have a simple construction and thus only low cost clocking devicesare employed, which has the disadvantage, that these clocks are subjectto a temperature drift to a clock inaccuracy drift, e.g. to a phasedrift, and/or to a frequency drift of the clock, which imposes a certaininaccuracy of the clocking of the individual stations AP1 to AP4. Theabsolute offsets and rate differences of the various clocks of thestations AP1 to AP4 may be quantified in microseconds of drift persecond, or equivalently, parts per million PPM. These offsets can beaccumulated or built up and maintained over long periods of many secondsor minutes when they are measured. This provides accurate measurement.The accumulated offsets or any related value thereto can be stored, e.g.in an optional location engine. The location engine may be implementedin the form of a server performing the calculation of the timedifference of arrival at the various stations.

For the time difference of arrival measurement, a time difference of thesignal flight times between a user device (not shown), which may beimplemented as a mobile device, and two or more receiving stations, forinstance AP2, AP1, and AP4 may be calculated.

In FIG. 1 hyperbolas marked from 2.410 to 2.480 indicate lines ofconstant delay differences between AP2 and AP4 whereas hyperbolas markedfrom 1.410 to 1.480 mark lines of constant delay difference betweenstations AP1 and AP4. As indicated by 110 in the given example theuser's position should be somewhere on hyperbola 120. On the other handas also indicated by 140 the user's position should also be somewhere onthe hyperbola marked by reference sign 130. Therefore, at theintersection of hyperbolas 110 and 130 the user's position alsoindicated by reference sign 140, respectively the position of the mobiledevice of the user, can be identified.

The embodiment of the present invention exploits the fact, that thedistances between the known stations AP1 to AP4 are known, as they arein a fixed positional relationship and that based on the separationdistances of the stations between each other, an accurate propagationtime of a correspondingly transmitted signal can be calculated by takingthe distance and the propagation speed. By establishing a timedifference of arrival based on an accurate calculation allows theestablishment of a relationship of this accurate time difference ofarrival to a time difference of arrival that has been measured by theinaccurate clocks of the two stations in question. Based on a model ofthe time dependency of the time difference of arrival at the tworeceiving stations, an analysis can be made to identify a correspondingerror value in the time difference of arrival measured by the inaccurateclocks of the two receiving stations. Further, based on the accuratecalculation the system is able to compute a correction value and thus toderive an accurate time difference of arrival at an arbitrary point intime regarding these two receiving stations. For instance, this can beachieved by relating the correct values at an arbitrary point in time tothe modeled value at this point in time to calculate the currentcorrection value. Mathematical operations in this context are preferablydivision to relate the values and multiplication to calculate thecorrected time difference of arrival for a signal received from a userdevice with the current correction value.

Such a process may be performed for any two receiving stations that arepart of the configuration shown in the embodiment of FIG. 1.

Consequently, according to the present invention no absolute values arerequired to be determined and instead only the relative clock behaviorbetween any two receiving stations needs to be determined. Of course thetime difference of arrival can also be determined by a user device,which on the other hand requires a number of further provisions forreceptions and forwarding of the corresponding data to and/or from theuser device. In order to perform an accurate location estimation of auser device a timing accuracy in the order of 1 ns which corresponds toone third of a meter is preferably required.

FIG. 2 gives an example of a clock behavior at two stations AP1 and AP2indicated by reference sign 290. On the vertical axis the local time ofthe clock at station AP2 is marked by 275 and on the horizontal axis thetime at station AP1 is marked by 265. The triangles in the diagramindicated by reference signs 215, 220, 230, 235, 240, 245 and 255represent the reception times of unique signals at the station AP1respectively AP2. If the clocks at AP1 and AP2 were performing correctlyand very accurately, there would be no curve connecting the trianglesand they would all lie on a straight line.

In this case the known propagation time of the signal based on acalculation of the signal propagation speed and the distance betweenrespectively AP4 and AP2 and AP4 and AP1 have been subtracted from thereception times of the signal, which may be a beacon signal from AP4. Asthe locations of the station are known such a calculation can be easilyperformed by a skilled person. It is also conceivable, that the curverepresented by reference sign 210 connecting the various triangles canbe modeled by any suitable regression or interpolation method, e.g. by asimple polynomial. However, other suitable modeling techniques areconceivable like value approximation by a neural network trained withthe value pairs of measured local clocks. Reference sign 250 indicatesthe time as an arbitrary point in time at which a signal from a mobiledevice of a user is received at the respective stations AP1 and AP2. Asmooth curve between measurements can be obtained by polynomial curvefit which yields a time difference between AP1 and AP2 at a userobservation time.

Such a dependency of the inaccurate local clocks is for instance furtherexplained in the configuration shown in FIG. 3. In particular here alocal clock of station AP2 referenced by 321 and a local clock ofstation AP1 referenced by 311 is shown. It is further indicated, that asignal propagation time between AP4 and AP2 marked by reference signT_(p4,2) is known based on a calculation of the distance and thepropagation speed, which also holds true for the propagation time of asignal transmitted between AP4 and AP1 indicated by T_(p4,1). Themeasurements taken for a signal transmitted from station AP4 and theassociated timing can for instance be transmitted to a server indicatedby reference sign 380 performing the required calculations as well as asuitable modeling function such as a regression analysis of which apolynomial curve fit is one example whereas the transmission isexemplified by an arrow marked by reference sign 350. For instance, arelative clock behavior may be obtained by over air measurements.

For instance, a propagation time correction may be performed in thefollowing manner. It may be assumed, that there are two receivingstations AP1 and AP2 at known locations. The clocks of AP1 and AP2 areticking in nano seconds and are not synchronized with each other. Thusthey are drifting in time with respect to each other. On the other handboth AP1 and AP2 are each able to accurately time stamp the samereceived signal for instance in form of a frame from a singletransmitting station AP4. It is known based on the signal propagationspeed and the distance between the stations that a frame transmitted byAP4 propagates 0.3 m in 1 ns and thus AP4 being 30 m apart from AP1 and18 m apart from AP2 leads to a propagation delay from AP4 to AP1 whichis 30 m/0.3 m/ns=100 ns whereas the propagation delay from AP4 to AP2 is18 m/0.3 m/ns=60 ns. For instance, AP1 time stamps a frame received fromAP4 with the reading of 629,154,927 ns whereas AP2 time stamps the sameframe received from AP4 with the reading of 402,549,572 ns. In this caseAP1 for instance calculates that its clock reading at the same time thatAP4 transmitted the frame was 629,154,927−100=629,154,827 ns, whereasAP2 calculates that its clock reading at the same time that AP4transmitted the frame was 402,549,572−60=402,549,512 ns. This leads tothe result that at the instant that the clock of AP1 was reading629,154,827 ns the clock of AP2 was reading 402,549,512 ns and viceversa. The calculation of a known propagation delay can however besubtracted at any time in the calculation process because it remains aknown constant in case the position of the stations remain constantwhich is a prerequisite. Thus a different order in the sequence of thecalculations is always possible. The fitting of a regression curve suchas a polynomial curve to an observed relative AP clock behavior may beperformed in the following manner. Preferably, a relative clock behavioris monitored over a segment of time containing n observations that spanthe instant when a relative timing of a user observation needs to beestablished. For instance, if the recorded n observation times, whicheventually need to be corrected for known propagation times of the samebeacon transmissions, are indicated by T_(APS,i) and T_(AP2,i), apolynomial curve may be fitted to the observed data which is, forexample, a least squares fit to the observed data, e.g. in form of

T _(AP2) =a ₀ +a ₁ T _(AP1) +a ₂(T _(AP1))².

Then the coefficients a₀, a₁ and a₂ can readily be obtained from theobserved data using a least squares matrix technique by:

a=(X ^(T) X)⁻¹ X ^(T) Y

where a is a column vector consisting of a₀, a₁, and a₂, Y is a columnvector of the n AP2 observations of T_(AP2,1) to T_(AP2,n) and X is ann×3 matrix formed from the AP1 observations e.g.:

$X = \begin{pmatrix}{1,} & {T_{{AP}\; 11},} & \left( T_{{AP}\; 11} \right)^{2} \\{1,} & {T_{{AP}\; 12},} & \left( T_{{AP}\; 12} \right)^{2} \\\vdots & \vdots & \vdots \\{1,} & {T_{{AP}\; 1n},} & \left( T_{{AP}\; 1n} \right)^{2}\end{pmatrix}$

Further details can be obtained from according tohttp://mathworld.wolfram.com/LeastSquaresFittingPolynomial.html.

In this example a second order polynomial is used wherein: a₀ representsa fixed time offset; a₁ represents a fixed frequency offset and a₂represents a linear frequency drift with time. Without restricting theinvention, it is also possible to use higher orders of polynomialsalthough a second order solution is likely to be good enough giving theshape of the curve shown in FIG. 2 which is not a complicated curve.However, there is a dependency between the order of the polynomial and anumber of measurement observations that are required for itsdetermination for a J^(th) order polynomial at least J+1 measurementobservations are required. Preferably in practice more measurementobservations are needed if the impact of measurement noise needs to bereduced.

FIG. 4 shows an example of a position determination process in aflow-chart according to an embodiment of the present invention.

At 405 the process is started. At 410 each station measures itsinaccurate signal arrival time against its own clock for every signal itreceives. At 415 in this embodiment the stations send the observationsmeaning the arrival time of the individual signal and the associatedsignal identification for instance to a location server which may beemployed to calculate a location. At 420 in this case the server storesall measurements over a period of time, for instance for the period ofsome 10s of seconds. At 425, in case of a location request (oralternatively periodically), each station receiving a suitable signale.g. a user frame, measures the arrival time of each particular userframe, representative of a user signal using its local clock and at 430sends the observed user signal measurement and the associatedidentification as well as time to the location server. At 435 the serverretrieves the values stored for signal transmission between the wirelessstations corresponding to the wireless stations that have received theuser signal and at 440 corrects the observation regarding the retrievedcommunication between the wireless station by subtracting the knownpropagation delays based on propagation speed and positionalrelationship, respectively known distances. Subsequently at 440 aregression analysis such as fitting a polynomial is carried out on thecorrected value pairs for each pair of wireless stations which receivedthe common intercommunication between the wireless stations. At 450 theserver, for instance, uses the polynomials to estimate the clock offsetsof the wireless stations at times corresponding to the time where theuser signal was received. At 455 for each pair of wireless stations at460 a time difference of arrival for the user signal is calculated andcorrected for the factor determined on the basis of the stored valuesfrom the corresponding pair of wireless stations and the calculatedpolynomial estimate of the clock offset. At 465 the corrected timedifference of arrivals information are transmitted to the locationalgorithm and at 470 the estimated user location is calculated byintersecting the two hyperbolas as shown in FIG. 1 but marked byreference sign 120 and 130 to determine the user location indicated byarrow 140 which is retrieved at 495. Marked by 475 such a process may berepeated to determine various user locations.

FIG. 5 shows an example of a computer program product according to thepresent invention. Reference sign 500 indicates a data carrier whichcomprises a program code 520 representative of any of the method stepsaccording to the present invention. Such a computer program productconstitutes a simple entity to transport the method of the presentinvention and to implement it on the transmitting and receiving stationsAP1 to AP4 of the present invention in case they are equipped with anetwork interface or a corresponding data reader. The data carrier ofthe present invention may be a suitable data carrier such as a magnetic-or optical medium or a hardware storage medium such as a flash storage.It may also be represented by a signal that is transmitted on a networkaccording to a certain network protocol implemented on a wired networkor on a wireless network for downloading the program code from onecomputer to another computer.

FIG. 6 shows an example of a station that may be employed in anarrangement according to the present invention. Reference sign 600indicates a wireless station e.g. an access point or any other wirelessor wire bound device capable of transmitting and/or receiving uniquesignals preferably identifiable by a unique identifier such as e.g. aframe number. It has an input/output interface 610 of any kind be itmechanical electrical or user specific for data entry and display.Further the station contains a receiver 615 and a transmitter 620 forinstance capable of emitting and receiving in the GSM, Bluetooth or WLANrange of frequencies, or to handle standard packet communication overwire or optical media. A controller or processor 625 is capable tocontrol the functions of the station 600 and has the power to performthe required computations. Further a memory 630 is present which may beany optical semiconductor or magnetic device for storing communicationand operation data. Reference sign 635 indicates a power supply whichmay be a battery or a transformer connected to a wall outlet. All theinternal components are connected by a suitable system bus 650 ensuringthe proper operation of the station 600. In a similar manner as thestation 600 a computation server 380 may be equipped with one or all ofthe components 610, 615, 620, 625, 630 and 650 dimensioned in acorresponding manner to perform a location determination of a userdevice and the required communication.

1. Method for clocking involving at least a first device (AP4), a seconddevice (AP2) and a third device (AP1) in a fixed positionalrelationship, the method comprising: at least the first device (AP4)transmitting at least a first and a second signal having a uniqueidentification; at least the second (AP2) and third device (AP1)associating a reception time measured by a respective second and thirdlocal clock (321, 311) of the second and third device to respectivelythe first and the second signal upon reception thereof, leading to atleast two value pairs of local clocks of the second (AP2) and third(AP1) device; based on the value pairs of local clocks, modeling a timefunction of a dependency of the second and third local clock over timeas a first model curve; at an arbitrary point in time receiving a usersignal from a user device at the second (AP2) and third (AP1) device andassociating a reception time measured by the respective second and thirdlocal clock (321, 311) of the second and third device to the user signalupon its reception, leading to a user value pair of local clocks of thesecond (AP2) and third (AP1) device; calculating a reference timedifference of arrival RTDOA for a signal from the first device (AP4)received at the second (AP2) and third (AP1) device from the fixedpositional relationship, based on the respective distance between thefirst (AP4) and the second (AP2) device and the first (AP4) and thethird (AP1) device and the known signal propagation speed; calculating auser time difference of arrival UTDOA from the user value pair; at thearbitrary point in time determining a value from the first model curveand based on this value calculating a determined time difference ofarrival DTDOA; relating the RTDOA to the DTDOA to determine a currentcorrection factor; and for clocking using the current correction factorto correct the UTDOA.
 2. Method according to claim 1, wherein more thantwo signals are transmitted; and wherein a respective model curve ismodeled for all signals.
 3. Method according to claim 1, wherein thesignals are transmitted over the air.
 4. Method according to claim 1,wherein the signal is a beacon signal.
 5. Method according to claim 4,wherein the signal is a frame.
 6. Method according to claim 5, whereinthe unique identification is a frame number.
 7. Method according toclaim 5 wherein the frame is a frame according to the standard IEEE802.11.
 8. Method according to claim 2, wherein the modeling of the timefunction involves a least squares fit of a polynomial.
 9. Methodaccording to claim 1, wherein at least one signal is being transmittedbefore the arbitrary point in time and at least one thereafter. 10.Method according to claim 2, wherein the same number of signals is beingtransmitted before and after the arbitrary point in time.
 11. Methodaccording to claim 1, involving a second third device in a fixedpositional relationship with the first to third devices (AP4, AP2, AP1)and modeling a second model curve and calculating a second user TDOAregarding a constellation of the second third device and the seconddevice (AP2) or the third device (AP1).
 12. Method according to claim11, for determining the location of a user device based on the first andthe second user TDOA.
 13. Arrangement for clocking comprising: at leasta first (AP4), a second (AP2) and a third (AP1) device; the first device(AP4) having a transmitter (620) for transmitting at least a first and asecond signal with a unique identification; the second and third devices(AP2, AP1) having a receiver (615) and a clock for receiving the atleast first and second signals and a controller (625) for associating arespective measured second and third local clock (321, 311) at thereception time of the respective signal to it, leading to at least twovalue pairs of local clocks of the second (AP2) and third (AP1) device;a processor (625) for modeling based on the value pairs of local clocksa time function of a dependency of the second and third local clock overtime as a first model curve; the second (AP2) and third (AP1) devicefurther being adapted to receiving at an arbitrary point in time a usersignal from a user device at the second (AP2) and third (AP1) device andassociating a reception time measured by the respective second and thirdlocal clock (321, 311) of the second and third device to the user signalupon its reception, leading to a user value pair of local clocks of thesecond (AP2) and third (AP1) device, the processor (625) being adaptedto calculate a reference time difference of arrival RTDOA for a signalfrom the first device (AP4) received at the second (AP2) and third (AP1)device from the fixed positional relationship, based on the respectivedistance between the first (AP4) and the second (AP2) device and thefirst (AP4) and the third (AP1) device and the known signal propagationspeed; and to calculate a user time difference of arrival UTDOA from theuser value pair; the processor (625) further being adapted to determineat the arbitrary point in time the value form the first model curve andbased on this value to calculate a determined time difference of arrivalDTDOA; wherein the processor (625) is further adapted to relate theRTDOA and DTDOA to determine a current correction factor; and forclocking to use the current correction factor to correct the UTDOA. 14.Arrangement according to claim 13, comprising a server (380) equippedwith the processor (625) to perform calculations and modeling connectedto the devices (AP1, AP2, AP4).
 15. Arrangement for clocking comprising:at least a first (AP4), a second (AP2) and a third (AP1) device; thefirst device (AP4) having transmitting means for transmitting at least afirst and a second signal with a unique identification; the second andthird devices (AP2, AP1) having receiving means and a clock forreceiving the at least first and second signals and processing means forassociating a respective measured second and third local clock (321,311) at the reception time of the respective signal to it, leading to atleast two value pairs of local clocks of the second (AP2) and third(AP1) device; modeling means for modeling based on the value pairs oflocal clocks a time function of a dependency of the second and thirdlocal clock over time as a first model curve; the second (AP2) and third(AP1) device further being adapted to receiving at an arbitrary point intime a user signal from a user device at the second (AP2) and third(AP1) device and associating a reception time measured by the respectivesecond and third local clock (321, 311) of the second and third deviceto the user signal upon its reception, leading to a user value pair oflocal clocks of the second (AP2) and third (AP1) device, processingmeans (380) for calculating a reference time difference of arrival RTDOAfor a signal from the first device (AP4) received at the second (AP2)and third (AP1) device from the fixed positional relationship, based onthe respective distance between the first (AP4) and the second (AP2)device and the first (AP4) and the third (AP1) device and the knownsignal propagation speed; and for calculating a user time difference ofarrival UTDOA from the user value pair; means for determining at thearbitrary point in time the value form the first model curve and basedon this value calculating a determined time difference of arrival DTDOA;wherein the processing means (380) is adapted to relate the RTDOA andDTDOA to determine a current correction factor; and for clocking to usethe current correction factor to correct the UTDOA.
 16. Arrangementaccording to claim 15, comprising a server (380) connected to thedevices (AP1, AP2, AP4) to perform calculations and modeling. 17.Computer program product (500) comprising a storage medium (500) onwhich a program code is stored, which when read and executed by acomputer performs the method according to claim 1 as process steps.